Thinking of this from a basic statics class standpoint they always make a general assumption in that first statics class that a car or any object is treated as a "Rigid Body" Meaning there is no flex in that structure. That way inputs or forces to the rigid body can be calculated with exact results to the "Rigid Body."

There are many books available out there for vehicle dynamics and chassis tuning,,,,anyone check into this one.

http://www.amazon.com/Vehicle-dynamics-damping-Jan-Zuijdijk/dp/1449059163

Here is a ppt from a Ford presentation at a conference regarding chassis rigidity and vehicle dynamics: http://www.altairhtc.com/europe/EHTC2011...2%20eteipen.pdf

There is a ton of information out there on chassis stiffening and rigidity. Like was said before the best way to do this is from an engineering stand point with a 4 post shaker and figuring out exactly where to add the extra weight when you need to do more than just seam weld the rockers, floors etc etc. A few points on the Mopar unibody are a given: Subframe connectors and torque boxes are the starting point to connect everything together and go from there. Where to stop just before you make your A body the same weight as a 57 Imperial.

Speaking of that remember the one article was done on a C body or was it an Imperial where they basically took all the doors etc sawzall'ed off the roof etc etc of this large Mopar to see how fast it would go 1/4 mile with half the car laying on the ground in the pits?